It is also true of turkeys. A hen has cells in her ovary that can develop into ova (eggs), and they are finite in number. Most birds have only one ovary. Mammals have two.
Everything about a bird has to be lightened for flight, and considering the relatively brief lives of birds, dispensing with one of the ovaries was a choice nature made when birds evolved eons ago.

A few bird species — some hawks, for example — have two ovaries, but only one is functional at a time.